The fountain, where the north tower of the World Trade Center once stood, was undergoing tests of the massive pumping systems that will blast 26,000 gallons a minute over the 30-foot deep black granite walls.

Daniels, who happened upon the test yesterday, said the sight of the water filling the 1-acre pool was awe-inspiring after years of planning and building.

Construction workers from around the site paused to look and listen during the tests as engineers below the memorial plaza monitored and adjusted the 16 pumps that will circulate 480,000 gallons of recycled water.

During the design phase of the memorial, there were concerns that the fountains might be too loud. But Daniels said the sound of the cascading water yesterday created “a nice, peaceful background, like the sound of the ocean.”

“It’s perfect. It’s not overwhelming at all,” Daniels said.

A second fountain over the footprint of the south tower will be ready for testing later this year. A spokesman for the Port Authority, which is overseeing construction of the $500 million memorial, said yesterday’s trial run went very well.

The two fountains together make up the nation’s largest manmade waterfalls, and are at the heart of the design of the memorial, called Reflecting Absence.

Paula Berry, a 9/11 family member who served on the 13-member jury that picked the memorial design by Michael Arad, was ecstatic about the test run.

“The presence of water is incredibly important at the memorial,” said Berry. “When you think of the memorial’s name, Reflecting Absence, you have to have the means to reflect and the water plays that role.”

But Berry said the sound of the water rushing over the sides of the fountains could very well be one of the most lasting impressions visitors take away.

Yesterday’s tests lasted a few hours, and will be repeated as needed.

Construction of the memorial remains on schedule for its planned opening next Sept. 11 — the 10th anniversary of the 9/11 terror attacks.

More than 50 of the plaza’s 400 trees have been planted so far.

The reflecting pools covering the exact footprints of the Twin Towers will be surrounded by the names of all the victims of the 2001 and 1993 attacks, etched into a bronze parapet that will be back-lit so the names can be read at night.

Arad’s design, which has been revised over the years in consultation with architect Peter Walker, was picked from an international competition that drew 5,201 submissions from 63 nations and 49 states.

Berry said watching the project come together leaves her believing more than ever that the right design was selected for the project, which will cover nearly half of the 16-acre World Trade Center site.

“The design has grown since it was first selected, and the various alterations and changes have just made it better,” she said.

Combined, the two fountains will circulate a total of 52,000 gallons of water per minute. Chemically treated like water in a swimming pool, and heated so that it can run through winter, the water will surge at four feet per second.

The entire volume of each fountain will circulate through the system every 22 minutes. And the fountains are designed to filter out coins in the likely event that visitors toss them into the pools of water.

Engineers yesterday were testing pumping pressure and making adjustments to ensure the correct flow of water through the fountains.

“The testing today went well, and it is an important sign of progress, but our work is by no means complete,” said PA Executive Director Christopher Ward.

“September 11, 2011, is the only goal that matters, and for the next 306 days, we will continue to keep our heads down and locked in on delivering the sacred heart of the site.”

Officially called the National Sept. 11 Memorial, the site will include a 9/11 museum below the plaza, scheduled to open a year after the memorial.